In offset lithography, a printable image is present on a printing member as a pattern of ink-accepting (oleophilic) and ink-rejecting (oleophobic) surface areas. Once applied to these areas, ink can be efficiently transferred to a recording medium in the imagewise pattern with substantial fidelity. In a wet lithographic system, the non-image areas are hydrophilic, and the necessary ink-repellency is provided by an initial application of a dampening fluid to the plate prior to inking. The dampening fluid prevents ink from adhering to the non-image areas, but does not affect the oleophilic character of the image areas. Ink applied uniformly to the wetted printing member is transferred to the recording medium only in the imagewise pattern. Typically, the printing member first makes contact with a compliant intermediate surface called a blanket cylinder which, in turn, applies the image to the paper or other recording medium. In typical sheet-fed press systems, the recording medium is pinned to an impression cylinder, which brings it into contact with the blanket cylinder.
To circumvent the cumbersome photographic development, plate-mounting, and plate-registration operations that typify traditional printing technologies, practitioners have developed electronic alternatives that store the imagewise pattern in digital form and impress the pattern directly onto the plate. Plate-imaging devices amenable to computer control include various forms of lasers.
Current laser-based lithographic systems generally rely on removal of an energy-absorbing layer from the lithographic plate to create an image. Exposure to laser radiation may, for example, cause ablation—i.e., catastrophic overheating—of the ablated layer in order to facilitate its removal. Accordingly, the laser pulse must transfer substantial energy to the absorbing layer. This means that even low-power lasers must be capable of very rapid response times, and imaging speeds (i.e., the laser pulse rate) must not be so fast as to preclude the requisite energy delivery by each imaging pulse. In addition, existing printing members often require a post-imaging processing step to remove debris generated during the imaging process.
As explained in U.S. Ser. No. 10/839,646, filed on May 5, 2004 and hereby incorporated by reference, a plasma polymer layer can be employed to facilitate selective removal of the imaging layer of a lithographic plate, which allows for imaging with low-power lasers. In addition, the printing member can be used on-press immediately after being imaged without the need for a post-imaging processing step. Although such plates are satisfactory for many applications, under some circumstances the oleophilic behavior of the exposed image areas can exhibit sensitivity to the power density delivered by the imaging sources. For example, power levels over 440 mJ/cm2 may cause thermal damage to the exposed image areas, compromising printing performance by reducing or even eliminating the oleophilic character of the substrate. It is found, for example, that plates incorporating plasma-polymer layers work very well with laser sources that provide a uniform (e.g., square and gaussian) energy profile, particularly at power density levels below 400 mJ/cm2, but may suffer performance degradation when imaged by laser sources that deliver a non-uniform (e.g., multimode) energy profiles. The reason is that, even at average power densities below 400 mJ/cm2, a multimode laser beam includes “hot spots” with energies well above the average, and which can thermally damage the plate. While it is possible to restore much of the lost printing performance through additional processing (e.g., cleaning with organic solvents, hydrophobic self-recovery by exposure to atmosphere for at least six hours, etc.), the extra steps involved and the environmental concerns posed by many solvents render such processing undesirable.